passive transport

Examples of passive transport in the following topics:

• The Role of Passive Transport

  • Passive transport, such as diffusion and osmosis, moves materials of small molecular weight across membranes.
  • The most direct forms of membrane transport are passive.
  • In passive transport, substances move from an area of higher concentration to an area of lower concentration .
  • The passive forms of transport, diffusion and osmosis, move materials of small molecular weight across membranes.
  • Diffusion is a type of passive transport.

• Transport of Electrolytes across Cell Membranes

  • Ions cannot diffuse passively through membranes; instead, their concentrations are regulated by facilitated diffusion and active transport.
  • The mechanisms that transport ions across membranes are facilitated diffusion and active transport.
  • All movement can be classified as passive or active.
  • Passive transport, such as diffusion, requires no energy as particles move along their gradient.
  • Active transport requires additional energy as particles move against their gradient.

• Facilitated transport

  • Facilitated transport is a type of passive transport.
  • The integral proteins involved in facilitated transport are collectively referred to as transport proteins; they function as either channels for the material or carriers.
  • Channels are specific for the substance that is being transported.
  • Channel and carrier proteins transport material at different rates.
  • Channel proteins transport much more quickly than do carrier proteins.

• Diffusion

  • Diffusion is a process of passive transport in which molecules move from an area of higher concentration to one of lower concentration.
  • Diffusion is a passive process of transport.

• The Plasma Membrane and the Cytoplasm

  • Passive (non-energy requiring) transport is the movement of substances across the membrane without the expenditure of cellular energy.
  • In contrast to passive transport, active (energy-requiring) transport is the movement of substances across the membrane using energy from adenosine triphosphate (ATP).
  • Active transport may take place with the help of protein pumps or through the use of vesicles.
  • Another form of this type of transport is endocytosis, where a cell envelopes extracellular materials using its cell membrane.
  • This is where a cell exports material using vesicular transport.

• The Respiratory System and Direct Diffusion

  • Diffusion is a slow, passive transport process.
  • Larger organisms have had to evolve specialized respiratory tissues, such as gills, lungs, and respiratory passages, accompanied by a complex circulatory system to transport oxygen throughout their entire body.

• Components of Plasma Membranes

  • The movement of a substance across the selectively permeable plasma membrane can be either "passive"—i.e., occurring without the input of cellular energy—or "active"—i.e., its transport requires the cell to expend energy.
  • The cell employs a number of transport mechanisms that involve biological membranes:
  • Passive osmosis and diffusion: transports gases (such as O2 and CO2) and other small molecules and ions
  • Transmembrane protein channels and transporters: transports small organic molecules such as sugars or amino acids
  • Endocytosis: transports large molecules (or even whole cells) by engulfing them

• Kidney Function and Physiology

  • Almost all nutrients are reabsorbed; this occurs either by passive or active transport.
  • Sodium (Na+) is the most abundant ion; most of it is reabsorbed by active transport and then transported to the peritubular capillaries.
  • Because Na+ is actively transported out of the tubule, water follows to even out the osmotic pressure.
  • Every solute, however, has a transport maximum; the excess solute is not reabsorbed.
  • Further up, Na+ is actively transported out of the filtrate and Cl- follows.

• Malpighian Tubules of Insects

  • Metabolic wastes, such as urea and amino acids, freely diffuse into the tubules, while ions are transported through active pump mechanisms.
  • There are exchange pumps lining the tubules which actively transport H+ ions into the cell and K+ or Na+ ions out; water passively follows to form urine.

• Electrochemical Gradient

  • Active transport mechanisms, collectively called pumps, work against electrochemical gradients.
  • Active transport maintains concentrations of ions and other substances needed by living cells in the face of these passive movements.
  • Two mechanisms exist for the transport of small-molecular weight material and small molecules.
  • Secondary active transport describes the movement of material that is due to the electrochemical gradient established by primary active transport that does not directly require ATP.
  • All of these transporters can also transport small, uncharged organic molecules like glucose.